1. Field of the Invention
The present invention relates to a shaft seal device to be used in for example a pump or other fluid machine, more particularly relates to a shaft seal device suitable when the sealed fluid is a fluid containing slurry or a high viscosity fluid etc.
2. Description of the Related Art
In a pump or other fluid machine, a shaft is provided rotatably in a casing. A shaft seal device is provided to prevent the fluid in the casing from leaking outside from the clearance between the shaft and the casing.
As a shaft seal device used in such a fluid machine, a mechanical seal is known. Sealing the clearance between a rotating shaft and a casing holding the shaft rotatively requires that a rotating seal ring rotating together with the shaft and a non-rotating seal ring fixed to the casing be brought into contact at the seal surfaces. At the seal surfaces, the wear between the rotating seal ring and the non-rotating seal ring becomes a problem. Therefore, recently, as shown in for example U.S. Pat. Nos. 5421593 and 4290611 and German Patent Publication DE3834214A1, a non-contact type mechanical seal provided with hydrodynamic grooves at the seal surface of the rotating seal ring and preventing the seal surfaces from contact together with rotation of the shaft has been proposed.
As the shape of the hydrodynamic grooves of the non-contact type mechanical seal, there are known spiral shaped ones as shown in U.S. Pat. No. 5071141 and T-shaped ones as shown in U.S. Pat. No. 5,092,612.
However, the dynamic pressure generating grooves of the non-contact type mechanical seal of the related art are formed at the outer diameter side of the rotating seal ring. Therefore, when using this non-contact type mechanical seal as a seal structure between the sealed fluid region and the purge gas region, the sealed fluid easily penetrates into the dynamic pressure generating grooves, so th at various problems occur to the seal surface induced by the sealed fluid. In particular, when the sealed fluid was a slurry or high viscosity liquid, there was the problem that the seal easily became incomplete due to wear at the seal surface.
Note that, as shown in U.S. Pat. No. 5,092,612, a non-contact type mechanical seal where hydrodynamic grooves are formed not only at the outer diameter side of the rotating seal ring, but also the inner diameter side is itself known. In the structure combining a plurality of mechanical seals into a system through purge region, using a mechanical seal formed with hydrodynamic grooves at only the inner diameter side of the rotating seal ring as the seal structure between the sealed fluid region and the purge gas region, however, was first proposed by the present inventors.
Further, U.S. Patent No. 5,421,593 proposes a seal structure using a contact type mechanical seal as the seal structure between the sealed fluid region and the purge gas region and making the pressure of the purge gas region lower than the pressure of the sealed fluid region. When using a contact type mechanical seal as the seal structure between the sealed fluid region and the purge gas region, however, the partial wear at the seal surfaces becomes severe and the durability becomes inferior. In particular, when slurry fluid is to be sealed, the wear at the seal surface becomes a problem. The sealed fluid leaks to the purge gas region having a low pressure and as a result may leak outside the casing. When the sealed fluid is a corrosive liquid, the seal components arranged in the purge gas region may be corroded and the durability of the seal device remarkably reduced.
In the seal structure described in this specification (U.S. Pat. No. 5,421,593), by just replacing the contact type mechanical seal with a conventional non-contact type mechanical seal, the pressure of the sealed fluid act as a back pressure onto the seal ring directly. Further, the sealed fluid penetrates into the hydrodynamic grooves formed at the outer diameter side of the seal ring, so that it induces the problem that the sealed fluid enters into the purge region having low pressure.
The present invention was made in consideration of this situation and has as its object to provide a shaft seal device with little wear at the seal surface, superior in durability, resistant to penetration of the sealed fluid into the purge region, and superior in sealability.
To achieve the above object, according to the present invention, there is provided a shaft seal device comprising:
a casing having a hollow portion,
a rotatable shaft extending inside the casing from a sealed fluid region to an outer region,
a non-contact type first mechanical seal arranged between the casing and the shaft so as to divide the hollow portion into the sealed fluid region and a purge region located between the sealed fluid region and the outer region, and
a second mechanical seal arranged between the casing and the shaft so as to divide the hollow portion into the purge region and the outer region, wherein
the first mechanical seal comprises a first rotating seal ring having a first rotating seal surface rotating along with the shaft and a first non-rotating seal ring held by the casing and having a first non-rotating seal surface able to be pushed axially and slidable against the first rotating seal surface,
a plurality of first hydrodynamic grooves communicating with the purge region formed at the first rotating seal surface or the first non-rotating seal surface so as to extend from an inner circumferential edge thereof to an outer circumferential edge direction, and
a purge fluid higher in pressure than the pressure of the sealed fluid fed into the purge region.
In the shaft seal device according to the present invention, it is possible to effectively prevent the entry of the sealed fluid into the purge region by the co-action between the purge fluid having a higher pressure than the pressure of the sealed fluid and the plurality of the first hydrodynamic grooves communicating with only the inner circumferential edge in the seal surface. That is, a sliding surface clearance is formed between the first rotating seal ring and the first non-rotating seal ring of the first mechanical seal by the action of the first hydrodynamic grooves. The first hydrodynamic grooves communicate only with the inner circumferential edge in the seal surface, so the purge fluid in the purge region is drawn into the first hydrodynamic grooves by the centrifugal force. Therefore, a barrier layer formed by the purge fluid is formed in the sliding surface clearance and it is possible to effectively prevent the sealed fluid from entering the purge region from the outer diameter side of the first mechanical seal.
Further, in the present invention, the first mechanical seal is non-contact type seal structure, so there is little wear of the seal ring and the durability is excellent. Further, due to an action of purge fluid spouting into the sealed fluid region etc., even if the sealed fluid is a slurry etc., an effective seal becomes possible.
Preferably, the pressure of the purge fluid in the purge region acts as a back pressure into a back surface of the first non-rotating seal ring located at an opposite side of the first non-rotating seal surface, and the first hydrodynamic grooves communicate with the purge region through an inner diameter hollow portion located at an inner circumferential side of the first non-rotating seal ring.
Because a back pressure of the purge fluid acts into a back surface of the first non-rotating seal ring, the first non-rotating seal surface of the first non-rotating seal ring is pushed axially against the first rotating seal surface of the first rotating seal ring. As a result, when a shaft doesn""t rotate, sealability between the seal surfaces improves.
Further, purge fluid acts into the inner circumferential surface of the first seal ring, so it is possible to make a configuration that easily introduces the purge fluid into the first hydrodynamic grooves.
Prefarably, the first rotating seal ring and the first non-rotating seal ring are made of a hard material in a hardness higher than that of a carbon. The hard material is not particularly limited, but may be SiC, tungsten carbide, a ceramic coated material, etc.
By making the first non-rotating seal ring using a hard material, its mechanical strength improves and even if the purge fluid acts from the inner circumferential side, harmful deformation doesn""t occur on the seal surface. Further,by making the first rotating seal ring and the first non-rotating seal ring by a hard material, it is possible to further reduce the wear of the seal ring as well as to prevent seal ring from deformation, and possible to improve the durability. In particular, even when the sealed fluid is slurry, it is possible to further reduce the wear of the seal ring.
Preferably, each of the first hydrodynamic grooves has an L-shape seen from the seal surface and comprises a first radial portion communicating directly with the purge region and a first circumferential portion communicating with an outer diameter portion of the first radial portion and extending in the circumferential direction and the mutually adjoining first hydrodynamic grooves are arranged line symmetrically.
By arranging the mutually adjoining first hydrodynamic grooves line symmetrically, even if the direction of rotation of the shaft becomes reverse, one of the first hydrodynamic grooves arranged line symmetrically functions and a sufficient clearance is formed between the seal surfaces. Note that the first hydrodynamic grooves are not limited to L-shapes and may also be T-shapes or spiral shapes.
Preferably, the second mechanical seal comprises a second rotating seal ring having a second rotating seal surface rotating along with the shaft and a second non-rotating seal ring held by the casing and having a second non-rotating seal surface able to be pushed axially and slidable against the second rotating seal surface, and a plurality of second hydrodynamic grooves are formed at the second rotating seal surface or the second non-rotating seal surface.
The pressure of the purge region is higher than the pressure of the outer region, so the purge fluid in the purge region enters to the second hydrodynamic grooves, forms a sliding clearance between the seal surfaces together with the rotation of the shaft, and forms a barrier layer of the purge fluid. Due to the purge layer, it is possible to seal the purge region and the outer region.
Preferably, the second non-rotating seal ring is constructed so that the pressure of the purge fluid acts as a back pressure into a back surface of the second non-rotating seal ring located at an opposite side of the second non-rotating seal surface.
Because a back pressure of the purge fluid acts into a back surface of the second non-rotating seal ring, the second non-rotating seal surface of the second non-rotating seal ring is pushed axially against the second rotating seal surface of the second rotating seal ring. As a result, when a shaft doesn""t rotate, sealability between the seal surfaces improves.
Preferably, one of the second rotating seal ring and the second non-rotating seal ring is made of a material mainly consisting of carbon. By making one of the mutual sliding members out of carbon, which is superior in slidability, the sealing property is further improved.
Preferably, each of the second hydrodynamic grooves has an L-shape and comprises a second radial portion communicating directly with the purge region and a second circumferential portion communicating with the inner diameter portion of the second radial portion and extending in the circumferential direction.
The mutually adjoining second hydrodynamic grooves are arranged line symmetrically.
By arranging the mutually adjoining second hydrodynamic grooves line symmetrically, even if the direction of rotation of the shaft becomes reverse, one of the second hydrodynamic grooves arranged line symmetrically functions and a sufficient clearance is formed between the seal surfaces. Note that the second hydrodynamic grooves are not limited to L-shapes and may also be T-shapes or spiral shapes.
Further, the second hydrodynamic grooves may be formed from the outer circumferential edge to inner direction or may be formed from the inner circumferential edge to outer direction. When it is formed from inner circumferential edge to outer direction, it is possible to maintain the pressure of the purge region for a long period.
Preferably, when the sealed fluid is a liquid, it is better the maximum depth of the first hydrodynamic grooves is larger than the maximum depth of the second hydrodynamic grooves. These depths are within an order of microns or ten microns.
When the sealed fluid is a liquid and the purge fluid is nitrogen gas or another inert gas, a seal is formed between the liquid and gas at the first mechanical seal and a seal is formed between the gas and gas at the second mechanical seal. In this case, by making the maximum depth of the first hydrodynamic grooves larger than the maximum depth of the second hydrodynamic grooves, it is possible to improve the sealing property between the gas and liquid at the first mechanical seal.
In the present invention, the sealed fluid is not particularly limited and may be a liquid containing slurry or a high viscosity liquid. Specifically, the sealed fluid may be a plating solution, a cleaning use chlorine solution, or other corrosive liquid and may be a hot monomer etc.